The present invention relates generally to blow molding operations and, more particularly, to a method and apparatus for rapidly measuring the volume of a mold to ensure that it falls within predetermined volume tolerances.
Blow molding has long been used for formation of hollow articles such as glass bottles. In blow molding operations a mold having left and right halves and a separate bottom is employed. The mold portions, when connected together, form a cavity having the shape of the article to be formed. The mold cavity has a relatively large opening therein for holding a parison which is used to produce a finished blow molded article. A compressed air source is attached at the cavity opening in sealed fluid communication with the parison in the mold cavity. The compressed air source forces air into the parison, causing it to expand into the mold cavity to form a hollow article in the shape of the mold cavity. As the parison expands into the mold cavity, air trapped in the mold cavity is vented from the cavity through small air passageways in the various mold portions. After the article formed from the blow molding process has cooled, the mold portions are separated and the article is removed.
In mass production bottle forming operations, the amount of material used to form a bottle and the volume of the mold cavity in which the bottle is formed must be very precisely controlled so that the volume of the finished bottle is uniform. Even a relatively small error in the control of these two parameters may result in a bottle having the appearance of being underfilled or overfilled with liquid at the completion of bottle filling operations.
The method of testing mold cavity volume which is the present industry standard requires grease to be applied to the interior of the mold cavity to cover the various air passageways in the mold which are used to vent air from the mold cavity during blowing of a parison. After grease is applied to the mold cavity air vents, the cavity walls must be carefully wiped down to remove excess grease. Next, the mold is clamped together and water in a beaker of known volume is poured into the mold cavity. When the water level in the cavity approaches the cavity opening at the top of the mold, the pouring process in stopped and water is thereafter added to the mold cavity by means of a syringe device. In this final stage of filling, water is removed from the beaker with the syringe and used to fill the mold cavity to the top. The remaining water in the syringe is then returned to the beaker. Finally, the water remaining in the beaker is carefully weighed to determine what volume of water has been used to fill the mold cavity. After the volume measuring operation has been completed, the grease in the air passageways of the mold cavity must be removed in a degreasing operation which generally involves immersion of the mold cavity in a degreasing agent and subsequent rinsing of the mold cavity. In a typical test lab, this method of measuring the volume of a mold cavity generally takes between 30 minutes and 1 hour and tends to be a somewhat messy operation due to the use of grease solvent, etc. It is also possible that the grease used to cover air vent passageways has a tendency to adhere to air bubbles formed when water is added to the mold cavity, and thus affects the accuracy of this volume measuring method.
It would be generally desirable to provide a mold cavity testing method and apparatus which enables a mold cavity volume test to be performed quickly, cleanly, and accurately.